With the rapid development of wireless communication technologies, there is an increasing demand for a low-power and high-speed radio-frequency transmission apparatus, for example, in application scenarios such as wireless capsule endoscopes, nerve recording etc. where a high data rate transmission is required.
Due to the constant envelope characteristics of frequency-shift keying modulation, a highly efficient nonlinear power amplifier can be utilized in a transmitter. Therefore, such modulation scheme is widely applied in scenarios where a low-power transmission is required.
Currently, transmitters available for frequency shift keying primarily comprise mixer structure-based transmitters and Phase-Locked Loop (PLL) structure-based transmitters. The mixer structure-based transmitters are highly flexible and can be used for different types of modulations. However, such structure requires high-power digital-to-analog converters and mixers, and therefore is not suitable for low-power applications.
The phase-locked loop structure-based transmitters are currently implemented in four ways on the whole. A first way is to apply a modulated signal to a frequency divider, which is simple and highly accurate. However, as a phase-locked loop presents low-pass filter characteristics for the modulated signal, a data transmission rate is limited by a loop bandwidth of the phase-locked loop. A second way is modulation with a closed-loop voltage-controlled oscillator, which directly applies modulated data to a locked voltage-controlled oscillator. In this case, a phase-locked loop presents high-pass filter characteristics for the modulated signal, and therefore low-frequency components of the modulated signal may be damaged. A third way is to add a modulated signal to a frequency divider and a voltage-controlled oscillator simultaneously, which is also called two-point modulation. This modulation way combines advantages of the two ways described above. In an ideal case, if a high-pass path exactly matches with a low-pass path, then a data transmission rate of a signal is not limited by a loop bandwidth of the phase-locked loop. However, the requirements for matching between both gains and bandwidths of the two signal paths increase the design complexity and power consumption of the system. A fourth way is modulation with an open-loop voltage-controlled oscillator, which firstly locks the voltage-controller oscillator at a certain transmission carrier frequency through a phase-locked loop, then disconnects the phase-locked loop, and directly applies a modulated signal to the voltage-controlled oscillator. In this way, a data transmission rate is free from limitations due to a loop bandwidth of the phase-locked loop. However, as the phase-locked loop is in an open-loop state during transmission, an oscillation frequency of the voltage-controlled oscillator is prone to frequency drift, which is generally caused by a leakage current, external interference and ambient temperature variation.